When a conference attended by many persons is held, a conference audio system is used, in which speaker's utterance is picked up by a microphone, amplified by an amplifier, and output from a speaker in a conference hall so that the speaker's utterance can be heard by all attendees. In such a conference that uses a conference audio system, many microphones are used. If many microphones are being turned on at the same time, that is, in an active state, audios picked up by these microphones are amplified and output from the speaker, and therefore, audios other than those of the speaker are heard as noises and the audio of the speaker cannot be heard clearly. In addition, a howl becomes more likely to occur. Because of this, a system has been widely used, in which an attendee turns on a microphone switch at hand when making a statement and turns off the switch after completing the statement. FIG. 6 shows a concept of the system.
In FIG. 6, on a table 1 in a conference hall, many microphones 11, 12, . . . , and in are arranged erectly on microphone stands 21, 22, . . . , and 2n. There are cases where each person uses each microphone and where two or more persons share one microphone. The microphone stands 21, 22, . . . , and 2n are each provided with a switch with which each microphone is turned on or off by the operation of an attendee. Audio signals from a microphone that has been turned on by the switch operation are input to a mixer 2 and the audio signals mixed in the mixer 2 are amplified by an amplifier 3 and the audios are output toward the attendees from a speaker 4 installed in the conference hall.
According to the above audio system, there occurs a time delay from when an attendee utters until the audio is converted into a signal in the microphone, mixed in the mixer 2, amplified by the amplifier 3, and output from the speaker 4. FIG. 7 shows the time delay. A waveform a represented by the solid line shows an attendee's utterance signal and a waveform b represented by the dotted line shows an audio signal from the speaker 4. As shown in FIG. 7, there occurs a time delay Δt between the waveform a and the waveform b. However, in a wired system as shown in FIG. 6, in which the microphone is turned on/off by manual operation, the time delay Δt is about 10 ms and there will arise no auditory problem because there is no auditorily uncomfortable feeling.
However, in the wired audio system as described above, it is necessary to connect all of the microphones with the mixer 2 by cables, and therefore, many cables are laid and it will be troublesome to handle the cables physically and to put them in order, and further, it will also be troublesome to identify the correspondence relationship between microphones and cables. The installation cost also becomes high.
Because of this, a conference audio system of cordless type as shown in FIG. 8 is proposed. In FIG. 8, the microphones 11, 12, . . . , and 1n stand respectively on microphone stands 31, 32, . . . , and 3n placed on the table. Each of the microphone stands 31, 32, . . . , and 3n incorporates a transmitter and transmits an audio signal converted in the microphone to a receiver 5. This transmission/reception system may be an optical communication system that uses infrared light etc., or a communication system that uses radio waves. The receiver 5 demodulates the received signal into an audio signal and the amplifier 3 amplifies the demodulated signal, and thus the audio is output from the speaker 4 installed in the conference hall toward the attendees.
On the other hand, in a system in which an on/off switch is attached to each microphone and an attendee needs to operate this switch, it may be troublesome to operate the switch and there may be a case where the attendee forgets to turn on the switch when making a statement or to turn off after his/her statement. Because of this, a conference audio system equipped with an automatic mute release device is proposed. In this system, an audio level detector is provided, which detects utterance or silence depending on whether or not the output level of each microphone exceeds a predetermined level, and normally, the microphone is put in an off state, that is, in a state of mute, and the audio level detector, when detecting utterance, turns on the microphone, that is, the mute is released. The automatic mute release device can also be applied to a wired system shown in FIG. 6 and a cordless system shown in FIG. 8.
In an elementary technique of an automatic mute release device, an audio level picked up by a microphone is detected and when the audio level becomes equal to or exceeds a predetermined threshold level (hereinafter, referred to as a “threshold”), the audio signal converted in the microphone is turned on. However, such an elementary technique of an automatic mute release device has a problem in that it takes time from when voice utterance enters the microphone until an audio signal is turned on, and the time delay Δt shown in FIG. 7 is about 100 to 200 ms and the beginning of speech may be lost.
As a technique to eliminate such a time delay, a method of automatically detecting beginning of speech is proposed, in which when an analog audio signal level from a microphone is equal to or greater than a threshold, a voice switch is turned on and while the voice switch is on, a digital recording circuit is activated and at the same time, the analog audio signal is input to a digital recording circuit for digital recording with a delay by a delay circuit corresponding to the maximum operation delay time when the voice switch is switched from the off-state to on-state (for example, refer to Patent document 1). The application of the technique described in Patent document 1 to a conference audio system will result in always causing a fixed time delay between when voice utterance is picked up by a microphone and when the audio is output from a speaker. Owing to this, there is no problem relating to the loss of beginning of speech. However, since both the words uttered directly by the speaker him/herself and the words of his/her own output from the speaker delayed in time are heard at the same time, the speaker will have an uncomfortable feeling. Further, since there is a discrepancy with respect to time between the movement of the lip of the speaker and the audio output from the speaker, the attendees other than the speaker also have an uncomfortable feeling. As described above, the time delay is always about 100 to 200 ms, and therefore, it is desired to solve this problem technically.
A recording device based on the same concept as that of the invention described in Patent document 1 is also known, which uses a tape recorder by an endless tape in place of a digital recording circuit (for example, refer to Patent document 2). The application of the invention described in Patent document 2 to a conference audio system will also result in the same problem as that encountered when the invention described in Patent document 1 is applied to a conference audio system.
Further, an audio communication recording device is proposed, in which an audio signal input from a microphone is converted into a digital signal and when the quantity of data stored in an FIFO buffer reaches a predetermined quantity, if there is no audio signal, the data is discarded and if there is an audio signal, the data is stored in the buffer or transmitted (refer to Patent document 3). The invention described in Patent document 3 states that it is possible to realize a natural conversation because a delay time between when the audio signal is received and when the audio is heard is short. However, when the invention described in Patent document 3 is applied to a conference audio system, if the audio signal is interrupted and it is determined that there is no audio signal, the audio data stored in the buffer is discarded, and when it is determined that there is an audio signal next time, the audio signal is stored sequentially from scratch and read out in order, and therefore, the effect to eliminate the delay in audio cannot be expected.    Patent document 1 Japanese Unexamined Patent Application Publication No. 60-163250    Patent document 2 Japanese Unexamined Utility Model Application Publication No. 60-142805    Patent document 3 Japanese Unexamined Patent Application Publication No. 8-265337